There has long been a recognized need for a mobile crane capable of lifting exceptionally heavy weights, as pointed out in U.S. Pat. No. 3,202,299, issued in 1965. A number of proposals have been advanced for satisfying this need, and most such proposals have involved the provision of a counterweight or auxiliary counterweight arranged to resist the forward tilting forces imposed upon the crane boom as a heavy load is picked up. In addition to the patent just mentioned, see for example U.S. Pat. No. 3,842,984, which discloses a trailer-like counterweight carrier that is pivotally connected to the boom carrying platform of a crane so that the counterweight carrier can have limited movement in a vertical plane. The last cited patent discusses other prior art that discloses counterweight and auxiliary counterweight trailers.
The several proposals in the prior art for counterweight arrangements are concerned with various aspects of the generally obvious problem of compensating for the forward tilting force that is imposed upon a crane boom when an exceptionally heavy load is imposed upon it. What has apparently not been appreciated heretofore is that high lateral tilting forces are also imposed upon the boom as the platform that carries it begins to rotate about its vertical axis and as it decelerates from such rotation. To afford optimum mobility, a mobile crane has the smallest base that will afford it adequate stability when it is lifting loads for which it is basically designed; hence there is no assurance that it will have adequate lateral stability when lifting the exceptional loads for which auxiliary counterbalancing is employed. It must be borne in mind that substantial and unexpected wind force loads can be added to the lateral forces imposed by a swinging load to increase the danger of lateral tilting.
The present invention is based upon a recognition of a relationship that has apparently not been appreciated heretofore, namely, that any substantial increase in forward tilting force upon the boom of a mobile crane, such as occasions the need for auxiliary counterweighting, is accompanied by a more or less corresponding increase in lateral tilting forces for which suitable compensation should also be made.
Thus it is the general object of the present invention to provide means for increasing the lateral stability of a mobile crane in substantially direct relationship to an increase in its lifting capacity that is obtained by equipping it with auxiliary counterweight means.
In the attainment of this objective, the maneuverability of the crane should not be compromised, and desirably no more than a minimum of auxiliary equipment--preferably none--should have to be installed on the crane in order to provide the increased lateral stability. In relation to these desiderata, it should be borne in mind that the augmented lateral tipping stability, to be useful, must be available in every position of rotation of the platform that carries the boom.
In view of these considerations, it is a more specific object of the invention to provide counterbalancing apparatus for resisting tilting forces on a mobile crane, adapted to be installed on the crane when an extraordinarily large load is to be hoisted with it, and whereby the crane is given increased resistance to lateral tilting forces in substantially direct ratio to increase in resistance to forward tilting forces.
Another specific object of the invention is to provide means in the nature of an accessory for a mobile crane, capable of being installed on it when it is to hoist an extraordinarily large load and whereby the crane is enabled to resist the tilting forces which the load imposes upon the crane in lateral as well as forward directions, and operative in all positions of rotation of the platform upon which the crane boom is mounted.